1. Field of the Invention
The present invention relates to a fixing device control method and apparatus, a fixing device, and an image forming machine, which are all suitable for use in a photoelectrographic copying or printing machine of the type equipped with a fixing unit in the form of a heat roller or the like and having a power saving function.
2. Description of the Related Art
In recent years an increasing number of image forming machines of the power-saving type have been put on the market which automatically assumes a power saving mode, after the lapse of a predetermined period without performing a image forming operation, to reduce the power consumption. In the meantime demands for such machines whose power consumption during the power saving mode is as small as to meet the criteria, such as, Energy Star.TM. are on the rise.
Generally in a photoelectrographic image forming machine, a fixing device consumes a great amount of electric power to heat toner of the image formed on printing paper into a molten form to fix it onto the paper. Consequently it is particularly effective to decrease power consumption of the fixing device itself during the power saving mode, which would result in reduced power consumption of the whole image forming machine during that mode.
In many of image forming machines of this type, the fixing device has a heat roller, with a built-in halogen lamp or the like, as a heating unit which melts unfixed toner of the image formed on the printing paper to fix it onto the paper.
In operation, the fixing device turns the halogen lamp on to heat the circumferential surface of the heat roller to a temperature high enough to melt the toner (target fixing temperature: 185.degree. C., for example) and then controls the temperature of the heat roller by turning the halogen lamp on/off.
While measuring the surface temperature of the heat roller using a temperature measuring device such as a thermistor, the fixing device turns the halogen lamp on/off so as to keep the temperature of the heat roller within the range of temperature enabling the fixing.
Specifically, the fixing device turns the halogen lamp on to heat the surface of the heat roller to the target fixing temperature, and then turns the halogen lamp off when the temperature reaches the target temperature. At that time, the surface temperature of the heat roller keeps increasing due to the remaining heat inside the heat roller, which would cause a phenomenon called overshooting: the surface temperature of the heat roller rises over the target fixing temperature.
Accordingly, it is important to appropriately design the heat roller (thickness or material of the roller, etc.) and to set the heat distribution characteristic of the halogen lamp such that the temperature of the heat roller would never exceed the temperature range enabling the printing operation, even if the temperature of the heat roller rises over the target fixing temperature.
After the halogen lamp has been turned off, the surface temperature of the heat roller overshoots the target fixing temperature and then gradually drops. When that temperature drops below the target fixing temperature, the fixing device turns the halogen lamp on again to heat the heat roller once more.
In the meantime, during recent years, printing machines have been used in a variety of environments. In an exemplified environment, a printing machine is coupled to a computer network and shared by the individual computers in the network. In use, the operators of two or more computers click the "start" at the same time to issue respective commands each for execution of a printing operation and such situation would often cause the printing machine to keep on stand-by for the printing operation for a long period.
As a solution, another power-saving printing machine is known in which electric power supplied to the printing machine is reduced during a stand-by mode for printing operation. After standing by a predetermined time without performing a fixing operation during the normal operation mode, a power-saving image forming apparatus of this machine shifts from the normal operation mode to the power saving mode to reduce power consumption. And a fixing device of the apparatus also shifts to the power saving mode accordingly.
Conventional methods for controlling turning on/off of the halogen lamp of the fixing device in the image forming machine during the power saving mode are exemplified by the following:
during the power saving mode, PA1 (1) turn the halogen lamp off; PA1 (2) keep the surface temperature of the heat roller around a reference temperature during the power saving mode, which is lower than the target fixing temperature, by turning the halogen lamp off when the current surface temperature of the heat roller is higher than the reference value during the power saving mode (100.degree. C., for example), and then turning the halogen lamp on when the surface temperature of the heat roller is lower than the reference value during the power saving mode; and PA1 (3) turn the halogen lamp on at a predetermined electric power.
FIG. 12 of the accompanying drawings shows tendencies of temperature rise of the circumferential surface of the heat roller when the temperature of the heat roller was controlled using four different target temperatures (25.degree. C., 155.degree. C., 160.degree. C., 165.degree. C.). As depicted in FIG. 12, it took a certain amount of period (approximately 100 seconds for each target fixing temperature) to heat the heat roller to the target temperature (target fixing temperature) at which the toner of the image can be melted).
However, none of the conventional control methods (1) through (3) considered a period needed for heating the heat roller to the target fixing temperature when the normal operation mode was restored after the power saving mode. Therefore, when the image forming machine restores the normal operation mode from the power saving mode, the period (restoration period) , during which the heat roller is heated to the target fixing temperature high enough to perform a printing (fixing) operation, would sometimes be longer than the user can be patient to wait thus resulting in less user-friendly products.
Yet, the above conventional control method (2) has another problem that repeatedly turning the halogen lamp on and off causes not only a shortened life of the heater lamp but also a sudden rise of electric current upon every turning on of the heater lamp, which would increase the power consumption.
Still another problem with the conventional control method (2) is that in an attempt to reduce the restoration period, the temperature of the heat roller during the power saving mode would tend to be set to a higher value than necessary, making it impossible to reduce the power consumption during the power saving mode so that only inadequate power saving results can be achieved.
Further, the restoration period depends on the temperature of the heat roller, which would tend to be influenced by the environment where the image forming machine is used, thus making it difficult to control the temperature.